This invention relate to degradation of products (including foam containers, such as foam coffee cups, and shaped packaging inserts for protection of electronic devices during shipment) formed from beads of expandable polymers, such as expandable polystyrene (EPS).
From an environmental viewpoint, a problem is created when such foam articles are discarded, because they have little tendency to degrade and ultimately disintegrate into beads or finer polymeric dust particles. It is believed that the undesireable cohesion of this plastic waste, in spite of the impact of solar radiation and other natural forces, arises for two reasons.
Firstly, a typical discarded foam article, such as a foam coffee cup, has integrity in the sense that sunlight reaches only the outermost portions of those beads which cover the outer surfaces of the foam article. In other words, polymer beads within the walls or other structures of the foam article, as well as inward-facing surfaces of outer beads (that define the outer surfaces of such foam article structures) are shielded from any potential degradation effects of direct solar radiation by the exposed outermost surface portions of those outer surface beads. Secondly, the solar radiation itself is not of the optimum wavelength for the promotion of polymeric disintegration. As is well known, the maximum intensity of natural solar radiation is about 290 nm (nanometers) to 330 nm by contrast, polystyrene degradation is most responsive to a electromagnetic radiation at a wavelength of approximately 260 nm (nanometers).
To deal with the first (shielding) of these difficulties, it is instructive to first understand the two available methods of pre-expansion (partial expansion) of polymer beads, such as EPS beads,
As is well known, pre-expansion (partial expansion) of EPS beads is a preliminary step in the manufacture of various articles, such as foam coffee cups, insulating coolers, shape-molded packing for electronic products and the like. For this purpose, pre-expanded polymer beads are introduced into a mold through a filling valve. Within the the mold, the loosely packed beads are caused to expand further until they largely fill the space between the beads and mold surface as well as the spaces between the beads, thereby forming the manufactured article, which can be removed from the mold after cooling. Such processing is disclosed in prior art publications, such as U.S. Pat. No. 3,897,899 issued to Schuff et al on Aug. 5, 1975.
For over 25 years, pre-expansion of EPS beads has been carried out by the first method of pre-expansion--using steam to provide the thermal energy to soften the unexpanded EPS crystal beads which contain a blowing agent such as pentane. Because steam is an efficient carrier of calories of thermal energy per unit mass, when intermixed with the crystal beads, it causes desirably rapid expansion thereof into much larger pre-expanded beads, each of which comprises a number of hollow cells. Each cell is formed of polymer cell walls, containing somewhat expanded pentane as well as water droplets from condensed steam. After a typically brief cooling period of less than an hour (to set the beads in their pre-expanded state) and a suitable "conditioning" period, the pre-expanded beads are next introduced into a mold for formation of a finished article. Conditioning also allows some of the pentane and condensed water vapour to escape through the cell walls to the surrounding atmosphere.
The purpose of the conditioning period (typically 8 to 24 hours) is both to allow the internal pentane pressure within cells and the atmospheric pressure to reach a steady state equilibrium (with concomitant increase in density) as well as to dry the pre-expanded beads sufficiently that condensed water vapour on the surfaces of these beads no longer agglomerates them into lumps that may not easily pass through the filling valve used in filling the mold or may not flow into corners and narrow spaces of the mold itself. Moreover, this conditioning period advantageously permits some of the water droplets (from condensed steam), that are inside the cells to escape through the cell walls thus drying the insides of the beads. Without inside drying, the trapped water droplets sometimes induce local non-uniformities in the molding of articles (e.g. holes in coffee cups) because each droplet requires longer heating to vapourize it before heating and expansion of the surrounding cell can progress. However, care must be taken that the conditioning period is not too long, as too much of the remaining blowing agent (e.g. pentane) may be lost by diffusion out of the cells of the pre-expanded beads, resulting in pre-expanded EPS beads that no longer have the ability to expand further when they are heated during molding. When the beads do not expand sufficiently during molding, the molded products tend to be poorly fused, and often crumble into pieces or, in the case of coffee cups, leak their contents. Thus, proper conditioning of EPS beads pre-expanded by the steam method has been a delicate balance between a sufficiently long time needed to dry the condensed steam introduced during pre-expansion, and a sufficiently short time to retain an adequate amount of blowing agent (e.g. pentane) within the pre-expanded beads.
As disclosed in our co-pending patent application entitled "Dry Expansion Of Expandable Polymer Beads" (Docket US-DPEP), it is now believed that the steam pre-expansion method functions by having the steam penetrate the crystal beads to carry a large thermal effect rapidly into the interior of each bead, where the steam condenses into water vapour, giving up most of its thermal energy by this change of state (at about 540 calories per gram of steam). In other words, it is the penetration of steam into the bead that allows efficient pre-expansion and formation of hollow cells containing thus-expanded pentane as well as water droplets from condensed steam, but this also causes the penetrated steam to condense into water vapor that is now deeply lodged within the cellular structure of each pre-expanded bead. Moreover, the interior cells near the center of each bead will be expanded by the thermal action of penetrated steam upon interior inclusions of blowing agent (e.g. pentane), thereby creating cells containing pentane which have thin interior cell walls that allow greater loss of pentane. This, in turn, gives rise to the delicate balancing required to maintain a conditioning period of the correct length.
According the second method for pre-expansion of EPS beads as disclosed in the aforementioned copending patent application, there is disclosed the use of a less thermally efficient transfer medium in the form of a dry heated gas, such as air, to more slowly pre-expand the crystal polymer beads. It is believed that the use of hot, dry (low water moisture content) air as a media for transferring heat to the crystal beads causes the beads to heat up from the surface towards the center. Due to this slower action by the heated air, which does not readily carry calories into the interior cell structure of the crystal beads (as compared to steam, which provides most of its calories by change of state, rather than conduction), a conductive type of heating of the bead takes place. Dry heated air, which provides no latent heat of condensation (unlike the 540 calories per gram provided by steam), only transfers about 0.24 calories per gram per degree Celsius of temperature difference between the EPS bead and the heated air. It is believed that the outer layer of each bead is first heated by the hot air and that layer by layer the heat penetrates conductively inwardly (both by infrared radiation and by permeation of dry heated air inward from peripheral toward interior cells, together hereinafter sometimes called "conductive heating"), thereby forming a more pre-expanded structure (with thinner cell walls) on the peripheral (i.e. outer) surface of each bead, and a less expanded structure (with thicker cell walls) at the interior (e.g. center) of each bead. As a result, the pre-expanded beads produced by air heating, contain a higher percent content of blowing agent (e.g. pentane) since it is locked within the interior cells of each bead by their surrounding thick walls of material which have not yet been heated by slow thermal conduction as much as the thin-walled peripheral (outer) cells. The resulting pre-expanded beads are advantageously very dry and can be used immediately in molding of containers and other EPS foam articles due to excellent flow characteristics and excellent expansion capability (due to high pentane content). Thus, the second ("dry air") pre-expansion method of our aforementioned copending patent application permits one to optionally dispense with the conditioning step in the preparation of EPS material for molding of foam articles. While degradation, according to the present invention, is applicable to both steam and dry air pre-expansion methods, it is preferable to work with the dry air method.